An inrush current elimination technique for a voltage sag compensator while powering transformer-coupled loads

Sags are the foremost power quality problem encountered by industrial consumers since they frequently causes the interruption of manufacturing processes that can result in significant economic loss. To resolve such problems, a voltage sag compensator is often installed by industrial consumers within critical industrial applications. However, the operation of the compensator takes a few milliseconds when a sag occurs. Hence the transformer, which is usually mounted in front of the critical load, is likely to be exposed to the sag voltages during this time interval. As a result, when the compensator reinstates the load voltage, a serious magnitude of transient current starts flowing towards the sensitive load. This inrush current can either damage the equipment or activate the protective devices of the compensator. In either case, compensation will eventually fail. To eliminate the inrush current phenomenon and to obtain seamless compensation for sensitive loads, this paper proposes a sag compensator based upon a customary current regulated voltage source inverter (CRVSI). This compensator employs a current control scheme implemented in a stationary frame of reference to regulate the current for the load by connecting it to the secondary of the load transformer. The operation of the proposed system is validated through laboratory test results obtained during different possible sag types.